This invention relates to magnetrons.
A known magnetron will now be described with reference to FIGS. 1 and 2 of the drawings. FIG. 1 is a fragmentary sectional view of the magnetron, taken through the axis of the anode, and FIG. 2 is an enlarged view of the anode and cathode shown in FIG. 1. Referring to FIG. 1, the magnetron comprises a vacuum chamber having end walls 1 and 2, which are at right angles to the axis of the anode 3 and cathode 4 of the magnetron. There are resonant cavities (not shown) defined in the anode, or by vanes. A magnetic field is applied normal to the plane of the end walls 1,2, by an electromagnet or a permanent magnet (not shown).
The cathode 4 is tubular, and has a heater extending along its axis, and a D.C. supply to the heater, as well as a high negative voltage for the cathode, is supplied to the cathode by means of conducting supports 5,6, which extend into an upper region 7 of the magnetron, the interior of which is within the vacuum envelope. The conducting supports connect to terminals on a part of the exterior of the upper region that is not shown. Couplers (not shown) extend into a resonant cavity and withdraw power into an output section 8, which can be coupled to a waveguide.
Referring to FIG. 2, which shows the tubular cathode 4 in more detail, a small DC supply voltage for the heater 9 is provided between the supports 5,6, and a large pulsed negative DC voltage is applied to support 6 only. The heater 9 is connected to the end of the tubular cathode at its right-hand end (as seen in FIG. 2), and the support 6 directly connects to the cathode. At its left-hand end, the heater 9 is supported in the cathode by an insulating sleeve 10, and is connected to the support 5.
The cathode is supported on radial arms 5, 6 that enable the magnetic field to be applied directly by a separate electromagnet (not shown). The gap over which the magnetic field is applied is desirably minimized so the electromagnet is as small as possible and uses least power. The vacuum gap between the ends of the tubular cathode structure and the end walls 1,2 of the magnetron has to be sufficient to hold off the negative voltage, typically 50 KV, that is applied to the cathode relative to the anode and the magnetron body, including end walls, under normal working conditions. Experience has shown that the cathode to end wall gap is not adequate to prevent arcing under all conditions (particularly when driven with line-type modulators) and very occasionally this can have serious consequences when the arcing causes the end wall to puncture. It is believed that in addition to the applied
pulse voltage across the cathode to side wall gap there are RF voltages picked up from where the cathode supports pass near the anode, particularly if there is a projection such as if the anode is provided with a strap 11. These picked-up voltages may be increased by resonances on the cathode supports or in the space between end plate and anode.
The heater connection 12 on the cathode is a sharp point, which further enhances the voltage stress in this area. The result is that the heater connection can form a seat for arcing, which can confine any arcing that occurs to the region of the wall 1 that is immediately adjacent, and thus increase the risk of perforation.
The Applicants contemplated counteracting this risk by the expedient of increasing the gap between side wall and cathode but this would mean the outline of the magnetron would need to change. However, there are thousands of equipments currently in use, which require the current profile for the magnetron, so such a modification would be disadvantageous. It is also undesirable to make anything more than the minimum change to the interior of the magnetron, as any change risks upsetting its operation.